Cannot set MYPYPATH

Thanks for this plugin - it's nice to have this running constantly inside PyCharm.

There may be a way to do this, but it's not in the README as far as I could tell, so I thought I'd bring this up.

For those of us who want to use types that exist outside our current project, it is nice to be able to `--follow-imports silent`. However this just leads to `handler.py:2: error: Cannot find module named 'botocore.vendored'` or whatever the module is.

When running from the command line, I can just add my VENV site-packages dir to the environment variable MYPYPATH and everything works great. However I'm not sure how to usefully define an env var for this plugin. More importantly, it really needs to be something that the plugin can figure out based on some kind of config file, e.g. the mypy.ini config file in the project root. However, setting mypy_path inside that config file does not seem to get picked up by this plugin.

Any thoughts on how to make this work properly?